1. Field of Art
The invention relates generally to mud pumps, and more particularly, relates to a cylinder liner system for mud pumps. Still more particularly, the invention relates to a renewable, compressive stress loading system for the cylinder liner of a mud pump.
2. Description of the Related Art
In extracting hydrocarbons from the earth, it is common to drill a borehole into the earth formation containing the hydrocarbons. A drill bit is attached to a drill string, and during drilling operations, drilling fluid, or “mud” as it is also known, is pumped down through the drill string and into the hole through the drill bit. Drilling fluids are used to lubricate the drill bit and keep it cool. The drilling mud also cleans the bit, balances pressure, and carries sludge and formation cuttings created during the drilling process to the surface.
Pumps, typically referred to as slush or mud pumps, are commonly used for pumping the drilling mud. Such pumps used in these applications are typically reciprocating pumps of the duplex or triplex type. A duplex pump has two reciprocating pistons that each force drilling mud into a discharge line, while a triplex reciprocating pump has three pistons that force drilling mud into a discharge line. These reciprocating mud pumps can be single acting, in which drilling mud is discharged on alternate strokes, or double acting, in which each stroke discharges drilling mud.
The motion of the reciprocating pump piston subjects the cylinder liner to reciprocating axial shear forces and cyclical internal pressures. The axial shear forces can lead to tensile stresses in the liner, and the cyclical internal pressures can lead to hoop stresses, both of which contribute to undesirable metal fatigue. To counteract the effects of fatigue, radial compressive stress pre-loading is often applied to the cylinder liner such that the cyclical internal pressures and associated hoop stresses are balanced by the pre-load compressive forces on the liner.
Most conventional mud pump cylinder liner systems include a housing and a sleeve coaxially disposed within the housing via an interference fit. The sleeve forms the inside surface of the liner and is typically made of a very hard and brittle material, such as chrome iron or ceramic. The radial interference fit between the housing and the sleeve generates the radially compressive forces acting on the sleeve, which serve to counteract the cyclical internal pressures and associated stresses. This conventional approach to counteracting fatigue is referred to as “pre-loading” since the radially compressive stresses are applied to the sleeve prior to its employment in the reciprocating pump (i.e., before the piston is axially reciprocated within the sleeve).
The pistons and cylinders used for mud pumps are susceptible to a high degree of wear during use because the drilling mud is relatively dense and includes a relatively high proportion of suspended, abrasive solids. As the cylinder in which the piston reciprocates becomes worn, the small annular space between the piston head and the cylinder wall may increase substantially, often in an irregular fashion. The flow of fluid through the annular space between the piston head and cylinder wall decreases the efficiency of the pump. To aid in reducing the effect of this wear, the cylinder is typically provided with a limited life, expendable cylinder liner.
The abrasive nature of the drilling mud translates into a relatively short lifetime for the cylinder liner and necessitates frequent replacement of the cylinder liner. Changing a cylinder liner in a conventional mud pump is typically a difficult, dirty, and costly job. For example, many conventional liner systems require replacement of the entire cylinder assembly including the liner, the housing, etc., which can weigh in excess of one-hundred pounds. In addition, access to the many of the parts involved in the cylinder liner replacement is limited, placing the maintenance personnel in awkward positions, increasing the potential for back or other physical injuries. Moreover, since drilling operations cease during mud pump maintenance, and drilling rig time is very expensive, frequent replacement of cylinder liners may be both inconvenient and costly.
Accordingly, there remains a need for improved systems, apparatus, and methods for installing and compressively loading cylinder liners that address the foregoing difficulties. Such improved systems, apparatus, and methods would be particularly well-received if they offered the potential to reduce the likelihood of injury to service personnel, minimize rig downtime, and simplified cylinder liner replacement procedures.